159.235 Graphics & Graphical Programming

1. 159.235 Graphics & Graphical Programming Lecture 32 - Illumination Part 2 - Global Models Graphics

2. Global Illumination • Extends the Local Illumination Model to include: • Reflection (one object in another) • Refraction (Snell’s Law) • Transparency (better model) • Shadows (at point, check each light source) • Antialiasing (usually means supersampling) Graphics

3. Wireframe View of a Test Scene Orthographic view from above Graphics

4. Test Scene High quality rendering of test scene. • Note : • Mirror and chrome teapot. • Shadows on floor. • Shiny floor. Graphics

5. Locally Illuminated Test Scene Ambient term only Graphics

6. Locally Illuminated Test Scene Phong shading. Ambient and Diffuse terms only • Notes : • Highlight on wall from light is in the wrong place; screen space interpolation. • We cannot illuminate the lights with the light sources – wrong side ! Graphics

7. Locally Illuminated Test Scene Phong shading. Ambient, diffuse and Specular terms. Graphics

8. Locally Illuminated Test Scene Flat shading. Note : Mach bands. Graphics

9. Locally Illuminated Test Scene Gouraud shading. Ambient, diffuse and Specular terms. Note: artefacts on wall. Graphics

10. Solution to Gouraud Artefacts Gouraud shading. Re-triangulated mesh. Graphics

11. Comparison Flat Gouraud Phong Coarser mesh Graphics

12. Use Local Illumination • No. • In our test scene, we can’t represent : • Mirror • Chrome teapot. • Shiny floor • Shadows with local illumination. Graphics

13. Kajiya’s Rendering Equation • Viewer is at point x, looking toward point x. • I(x,x) determines amount of light arriving at x from x • g(x,x) is a geometry term, = 0 when x is occluded, otherwise = attenuation factor 1/r2 (or 1/(s+k)) • (x,x) is the amount of light emitted from x to x. • (x,x,x) is the fraction of light reflected and scattered off x to point x from point x • The integral S is over all such points (x") on all surfaces. Graphics

14. Global Illumination • Two methods : • View dependent methods. • Calculate the view from the camera with global illumination. • Recursive ray-tracing. • View independent methods. • Solve lighting for the entire scene. • Radiosity solution. Graphics

15. View Dependent Methods • Loop round the pixels….. • Good for lighting effects which have a strong dependence on view location : • Specular highlights. • Reflections from curved surfaces. • Only a small number of objects need to be considered at the same time. • Poor when many objects need to be considered • E.g diffuse interactions (eg. colour bleeding). Graphics

16. View Independent Methods • Loop round the scene… • Good when many (all) objects need to be considered at same time. • Diffuse inter-reflections. • Poor when shading has strong dependence on view location. • Specular reflection. Graphics

17. Scene Eyepoint Window Recall : Ray Casting • Involves projecting an imaginary ray from the centre of projection (the viewers eye) through the centre of each pixel into the scene. • The first object the ray intersects determines the shade. Graphics

18. Whitted’s algorithm • Fire off secondary rays from surface that ray intersects. • Towards Light Source(s) : shadow rays. L (shadow feelers) • In the reflection direction : reflection rays, R • In a direction dictated by Snell’s Law : transmitted rays, T    Graphics

19. Recursive Ray Tree • Reflection and Transmission Rays spawn other rays. • Shadow rays test only for occlusion. • The complete set of rays is called a Ray Tree. Light Source ray determines colour of current object. Viewpoint Graphics

20. Recursive Ray Tree • Reflection and Transmission Rays spawn other rays. • Shadow rays test only for occlusion. • The complete set of rays is called a Ray Tree. Graphics

21. Test Scene Ray tree depth 1. Note only ambient shade on mirror and teapot Graphics

22. Test Scene Ray tree depth 2. Note only ambient shade on reflection of mirror and teapot. Graphics

23. Test Scene Ray tree depth 3. Note only ambient shade on reflection of mirror in teapot. Graphics

24. Test Scene Ray tree depth 4. Note ambient shade on reflection of teapot in reflection of mirror in teapot. Graphics

25. Test Scene Ray tree depth 5. Graphics

26. Test Scene Ray tree depth 6. Graphics

27. Test Scene Ray tree depth 7. Graphics

28. When to stop ? • Need to know when to stop the recursion. • Can define a fixed depth. • Hall introduced adaptive tree depth control. • Calculate maximum contribution of a ray to a pixels final value. • Multiply contribution of ray’s ancestors down the tree. • Stop when below some threshold, perhaps stack overflow. • May miss major contribution this way (culled bright pt) Graphics

29. Adaptive Tree Depth Control 0.3 Viewpoint 0.2 Graphics

30. Adaptive Tree Depth Control 0.3 0.3 * 0.2 Viewpoint 0.2 * 0.2 0.2 Graphics

31. Global vs. Local Illumination • In both an object hit by a ray, if lit by a light source, is illuminated by a local illumination model, i.e with specular, diffuse & ambient terms. • Global: a reflected ray, a shadow feeler, and a transmission ray (if appropriate) are also cast into the scene. • Phong term only reflects light source. • Need to adjust local illumination terms to normalise total light values. • Inconsistent if local and global specular terms used together as local term spreads light source, global term does not. Graphics

32. Increased reflectivity Phong specular term is held constant Increased transmissivity Graphics

33. Incorrect Result • Effect of not normalising reflection and transmission – light appears to be created. • Reflection & transmission = 100% Graphics

34. Problems with Ray-Tracing • A serious problem with Ray tracing is rays are traced from the eye. • Refraction is not physically correct. • Shadow rays are cast only to light sources • Lights reflected in mirrors do not cast shadows • Shadows of transparent objects don’t exhibit refraction. • Still need local illumination for diffuse shading. Graphics

35. Speeding up Ray Tracing • Ray tracing is slow, not real-time. • Use appropriate extents for objects. • Ray tracing is inherently parallel. • Use item buffers – z-ordered lists, store closest object per pixel. • Use light buffer – z-ordered list per light ray used for shadowing. Graphics

36. Illumination Part 2 - Summary • Reflection • Refraction • Transparency • Shadows • Antialiasing • Acknowledgments - thanks to Eric McKenzie, Edinburgh, from whose Graphics Course some of these slides were adapted. Graphics